1. Field of the Invention
The present invention relates to a honeycomb structure, which can be used as a preheater, a particulate trap, or a catalytic converter for controlling automobile exhaust emission. The preheater may be an element in a catalytic converter for controlling automobile exhausts emission, and a method of making the honeycomb structure.
2. Description of the Related Art
Conventionally, a porous ceramic honeycomb structure has been used as a carrier for a catalyst composition for removing, for example, nitrogen oxides, carbon monoxide, and hydrocarbons in the exhaust gas of the internal combustion engine of an automobile. Recently, a honeycomb structure composed of a metallic material has drawn attention since the metallic honeycomb structure has high mechanical strength and heat resistance. The metallic honeycomb structure has two different types. One of the them is a so-called foil-type honeycomb structure that has a corrugated thin metal foil. The other type is a so-called extruded-type honeycomb structure that has a unitary structure formed by sintering an extruded body comprising a powder material.
As restriction of exhaust emission controls has been intensified, there has been a demand for heaters adapted to decrease pollutants during the initial running of the engine when the engine is not warmed up. The removal of hydrocarbons, carbon monoxide, nitrogen oxides while an exhaust gas is cold is particularly important since the catalyst composition has a low activity for oxidizing hydrocarbons in the cold exhaust gas. Moreover, during the initial running of the engine, the engine emits a large amount of pollutants as compared with during the steady continuous operation. Hence, it is preferable to heat the exhaust gas during the initial running of the engine by a preheater to a temperature at which the catalyst composition is sufficiently activated so as to convert pollutants in the exhaust gas.
An apparatus for controlling an exhaust gas of the engine in the exhaust system of an automobile may includes a preheater and a main catalyst element in the exhaust system, and the preheater is provided upstream of the main catalyst element.
U.S. Pat. No. 5,063,029 discloses a preheater and a catalytic converter. The preheater includes an electrically conductive honeycomb structure, at least two electrodes in electrical contact with the honeycomb structure, and a slit for adjusting electrical resistance of the honeycomb structure.
U.S. Pat. No. 4,576,800 discloses a honeycomb structure for loading a catalyst composition for an exhaust gas. The honeycomb structure has a limited geometric specific surface area so that an amount of the catalyst composition loaded onto a surface is limited.
Japanese Patent Application Laid-Open No. 4-148016 discloses a foil-type honeycomb heater that has characteristic features in connection between a flat thin sheet and a corrugated thin sheet so as to increase resistance to a thermal stress so that the honeycomb structure does not deform or crack easily.
Japan Patent Publication No. 58-23138 discloses a metallic substrate having an extended honeycomb structure so as to increase its geometric specific surface area for loading a catalyst layer. However, the more surface area increases a purification rate of pollutants only when rates of catalytic reactions of the pollutants are limited by the catalytic reactions itself.
Japan Patent Application Laid-Open No. 4-203416 discloses a foil-type honeycomb heater including a corrugated heater and a flat heater wherein the corrugated heater is meshed so as to adjust its electrical resistance.
However, the aforementioned related art overlooks that a contact of an exhaust gas with a catalyst composition is a factor to determine a purification rate of the exhaust gas. Even though a geometric specific surface area of a catalyst layer increases and an exhaust gas is quickly heated to an activating temperature of the catalyst, the exhaust gas needs to have a contact with the catalyst composition while the exhaust gas passes through passages in the honeycomb structure, and the contact of the exhaust gas with the catalyst composition may be a rate-determining step.
A Reynolds number is a parameter for a state of a fluid flow, and the fluid flow is classified into three states: a fast flow, a medium flow, and a slow flow. The Reynolds number for an exhaust gas passing through a typical honeycomb structure in the exhaust system ranges from about 30 to about 340, and the range comes under the medium flow. In the medium flow, a viscosity of the fluid itself cannot be ignored unlike a fast flow, and a transfer of the fluid in directions perpendicular to the flow direction is limited compared with the fast flow.
On the other hand, the fast flow corresponds to the Reynolds number larger than 1000. In the fast flow, a viscosity of the fluid can be ignored, and the fluid flow is turbulent so that fluid sufficiently transfers in directions perpendicular to the flow direction.
Japan Patent Application Laid-Open No. 3-118838 discloses a catalytic converter containing a foil-type honeycomb structure. The honeycomb structure has passages having a non-linear shape along its axial direction, such as a zigzag pattern, so as to mix a fluid stream. However, the mixture by the passage shape may have its limitation.